Plasma membranes isolated from ethanol treated rat erythrocyte ghosts and brain synaptosomes have been used to study calcium binding. Terbium fluorescence and calcium-45 were used to probe the membrane calcium interaction in cerebellum, cortex and hippocampus of ethanol dependent and withdrawing rats. The fluorescence measurements with terbium in the presence and absence of exogenous calcium were used to determine the calcium dissociation constants (Kd) for the membranes. The Kd values obtained for the synaptosomal plasma membranes (SPM) isolated from different groups of animals were Kd (control) = 8.2 mM, Kd (dependent-intoxicated) = 6.9 mM and Kd (dependent-withdrawing) = 8.6 mM. No change in Kd was observed for animals treated with a single dose of ethanol when compared to controls. The results indicate an increase in the calcium binding affinity during the dependent-intoxication phase. The calcium-45 binding studies were performed on SPM prepared from cortex, hippocampus and cerebellum regions. The calcium-45 binding was found to be increased by 40 plus/minus 10% in dependent-intoxicated rats and 25 plus/minus 10% in dependent-withdrawing rats when compared to controls. Similar results were obtained in erythrocytes ghosts. Addition of exogenous ethanol to SPM caused a decrease in calcium-45 binding in control and dependent-intoxicated rats and an increase in binding in withdrawing animals. The results of this project suggest that a difference in calcium binding to plasma membranes is associated with ethanol dependence and withdrawal. The membranes from dependent-intoxicated animals were more resistant to the exogenous addition of ethanol, while dependent-withdrawing animals were less sensitive. This information provides an experimental basis for investigating the alterations of plasma membranes that are associated with dependence upon ethanol.